During cardiac arrest, the heart does not pump blood, and thus the brain does not receive freshly oxygenated blood. Irreversible neurological damage begins soon after circulation stops. Thus, it is imperative that oxygenated blood be continuously supplied to the brain and other vital organs by artificial means during cardiac arrest to insure that the patient will survive neurologically intact and without significant brain damage after resumption of cardiac function.
Conventional resuscitation techniques such as CPR, heart massage and electroshock treatments are notoriously inefficient in maintaining the supply of oxygenated blood within the body. Among cardiac arrest victims overall, less than 10% survive neurologically intact and without significant brain damage. Presently, the majority of patients die or sustain some neurological injury from ischemia (lack of blood flow to the brain) or anoxia (lack of oxygen to the brain). Additional disadvantages of the conventional techniques are discussed in the U.S. Pat. No. 4,424,806, the entire contents of which is incorporated herein by reference.
The reason the conventional techniques of resuscitation have such a high rate of morbidity and mortality are numerous, but are believed to be focused upon the following factors. Conventional resuscitation techniques typically supply only 25 to 30% of resting cardiac output and provide a mean arterial pressure (MAP) of only 30 to 45 mm Hg, compared to the 50 to 55 mm Hg MAP minimally required to maintain brain viability for even short periods of time.
A first factor in the inefficiency is that the lungs, which are normally gas-filled, form compliant compartments. As such, during CPR, the compartments act to dampen or decrease the effectiveness of the force applied as a compression to the chest. Thus, the intrathoracic pressure, i.e., the "thoracic pump," and not the force applied to the chest, determines the amount of cardiac output during CPR. The lungs act to decrease the intrathoracic pressure.
As a further factor, during normal heart operation, blood rich in oxygen and poor in carbon dioxide returns from the lungs to the left atrium and enters the left ventricle, which contracts and forces the blood into the aorta for distribution throughout the body. The flow is maintained by the mitral valve, which regulates the flow by opening and closing during contractions of the left ventricle. However, during conventional resuscitation techniques, the compression and decompression does not fully close the mitral valve. This results in regurgitation of the blood in the left ventricle into the heart and consequently, low cardiac output.
As a third factor, during CPR, the thoracic vena cava and the right atrium are compressed, resulting in an abnormally high venous pressure. This is related to a corresponding decrease in the volume of deoxygenated blood returning to the right heart. This results in an inadequate preload, i.e., inadequate volume of oxygen depleted blood in the heart, and thus a further reduction of the resultant cardiac output.
Finally, pulmonary edema, which results from a high pulmonary artery pressure and central venous pressure develops rapidly during conventional resuscitation techniques. This compromises gas exchange with the blood in the lungs and thus further reduces the efficiency of conventional resuscitation techniques.
It is in many instances also desirable to chill and/or warm patients during treatment. For example, patients suffering from sudden cardiac arrest, shock, severe hypotension, or ischemia may need rapid reduction in body temperature in order to reduce metabolic demands to levels capable of being provided for by compromised body systems or conventional resuscitation techniques, or otherwise to provide specific protection afforded to organs by rapidly inducing and maintaining hypothermia after an ischemic or traumatic event. Similarly, it is desirable to rewarm patients, for example, patients who no longer need a reduced body temperature or who have otherwise experienced accidental hypothermia.
Chilling and/or rewarming of patients is traditionally done externally to the body, for example by applying ice packs or with refrigeration. These methods produce very slow cooling. Newer methods involving cold intracarotid infusion or intraperitoneal infusion, etc. are faster, but would also be more effective still when combined with other cooling modalities. A simpler and less invasive cooling method that is consistent with current practices would be very beneficial.
U.S. Pat. No. 5,158,536 to Sekins discloses a method for treating lung cancer in which warm fluid is introduced into the lungs and then removed from the lungs in rhythmic patterns.